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by the difficulty of selecting which synapses to modify. The information that reaches the
medial temporal lobe is highly integrated and episodic memory research faces methodological
problems in decoding the informational content of hippocampal network. Conceivably, the
functional effects of synaptic plasticity are more tractable for other forms of memory in the
brain and particularly perceptual memory in sensory cortices.
Figure 10. Shifts in perceived orientation and spike timing-dependent modification of the intracortical
connections. A. The population profile of the orientation tuning response evoked by a visual test
stimulus is shown before (solid lines) and after (dotted lines) conditioning. The colored circles represent
the responses of a three neurons to the test stimulus, as predicted by their respective tuning curves. In
this scheme, the perceived orientations are determined by the peak positions of the population response
curves. Depending on the direction of the conditioning stimulus, the connections between the involved
neurons will be affected differently. According to the spike timing-dependent rules the synaptic
strength will increase if the presynaptic spike precedes postsynaptic activity and will decrease if the
presynaptic spike follows the postsynaptic activity. Similarly if the orientation of the stimulus activates
cell (a) after the cell (c), LTP of the projection from (c) to (a) will occur. At the same time (a) will be
activated pror (b), which will lead to LTD of the projection from (b) to (a). The gray arrows indicate the
temporal order of neuronal spikes. B. Test stimulus with the opposite orientation will activate the cells
in reverse order, which will result in LTD of the projection from (c) to (a) and LTP of the projection
from (b) to (a). The thickness of connecting lines represents the synaptic change, based on spike timing-
dependent strengthening and weakening of the intracortical connections (adapted from (Yao & Dan,
2001)).
Plasticity is an integral part of information processing in visual cortex. In general, since it
involves cortical areas at the early stages of visual processing, where most is known about
neocortical circuitry, receptive field properties, and functional architecture, these cortical
areas are therefore more tractable for learning the underlying mechanisms. Thus, the adult
primary visual cortex has been the most used model up-to-date for exploring the phenomenon
of neocortical synaptic plasticity, starting at the very early stages of visual processing
understanding (Wiesel and Hubel, 1963 ). A striking aspect of the findings related to
plasticity research is the apparent ability of the adult cortex to dynamically modify the
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